For generations, the Yashiro family upholds a peculiar custom of nurturing a mermaid's egg received every six decades, safeguarded in a shrine until it matures before returning it to the sea. This ritual ensures the town's prosperity, bestowed by the merfolk in gratitude. However, as urban developers threaten their quaint coastal village and Yosuke's father succumbs to the allure of profit, transforming the sacred egg into a tourist attraction, the family's pact with the mermaids hangs in the balance. Amidst this turmoil, the one-shot 'Summer of Tension' unravels the consequences of straying from age-old promises.